How to Import Another Python script using Python Script
Here's what I've just done, it seems to work.
My paths are:
scripts path -

C:\Users\Drew\Documents\Development\bfd-graphics\Blender Scripts

mainscript path -

C:\Users\Drew\Documents\Development\bfd-graphics\Blender Scripts\main.py

blender file path -

C:\Users\Drew\Documents\Development\graphics\3D
Assets\Prototype\Blender Projects\Templates\Builder.blend

I wanted to be able to import scripts from a completely different but known relative directory. So here is how I tackled it:
import bpy, sys
from pathlib import Path

path = Path(bpy.data.filepath)
sys.path.append(str(path.parent.parent.parent.parent.parent) + &quot;\\Blender Scripts&quot;)

import test_import

test_import.print_a_message()

The number of parents determines where in the directory structure I am, and then I add the rest of the path as a string.